SI-8019 Make Publisher check 'Reaction' partial-function is defined for an Event

Reactions are PartialFunctions, so if events come through indiscriminately that the listener is not defined for, errors occur. Eg:

```
Exception in thread "AWT-EventQueue-0" scala.MatchError: FocusGained(scala.swing wrapper scala.swing.TextField
```

A Coursera thread with people affected by this issue: https://class.coursera.org/reactive-001/forum/thread?thread_id=1315

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